This invention relates to an electrostatographic printing machine, and more particularly concerns a color electrophotographic printing machine wherein red, black, or red and black information may be formed on one or both sides of a copy sheet.
In electrostatographic printing an electrostatic latent image is created and reproduced in viewable form. The process of electrostatographic printing includes electrophotographic printing and electrographic printing. Electrophotographic printing employs a photosensitive medium to form, with the aid of electromagnetic radiation, an electrostatic latent image. Contrawise, electrographic printing utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent image. Hereinafter, an electrophotographic printing machine will be described as an illustrative embodiment of the inventive concept described in the present application.
In the process of electrophotographic printing, for example, as disclosed in U.S. Pat. No. 2,297,691 issued to Carlson in 1942, a photoconductive member is charged to a substantially uniform potential thereby sensitizing its surface. Thereafter, the charged photoconductive surface is exposed to a light image of an original document. In the irradiated areas, the charge is selectively dissipated in accordance with the intensity of the light image transmitted thereto. This records thereon an electrostatic latent image corresponding to the original document. Development of the electrostatic latent image is achieved by bringing a developer mix into contact therewith. A typical developer mix employs colored heat settable plastic particles known generally as toner particles, which are mixed with ferromagnetic granules, i.e. carrier granules. The developer mix is selected such that the toner particles acquire the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. As the developer mix is moved into contact with the photoconductive surface, the greater attractive force of the electrostatic latent image causes the toner particles to be separated from the carrier granules and adhere to the electrostatic latent image. The toner powder image adhering to the electrostatic latent image is, then, transferred to the sheet of support material. A suitable sheet of support material is paper, or a plastic sheet, amongst others. Subsequently, the toner powder image is permanently affixed thereto.
Essentially, multi-color printing repeats the foregoing process a plurality of cycles. For example, U.S. Pat. No. 3,531,195 issued to Tanaka et al. in 1970 discloses a multi-color electrophotographic printing machine. As recited therein, the light image is filtered to record an electrostatic latent image on the photoconductive surface corresponding to one color of the original document. The electrostatic latent image is then developed with toner particles complementary in color to the filtered light image. Thereafter, the toner powder image is transferred to the sheet of support material. The foregoing process is repeated for a successively different colored light image. In this manner, a plurality of toner powder images are transferred to the sheet of support material, in superimposed registration with one another. As described in Tanaka, each toner powder image is fused after being transferred to the sheet of support material.
Since the advent of electrophotographic printing, various machines and devices have been developed to incorporate the teachings thereof in a manner to create simplex and duplex copies on a commercial basis. For the most part, these machines are limited to making a specified number of copies from an original on one or both sides thereof. For example, U.S. Pat. No. 3,592,462, 3,630,607 and 3,615,129 are all directed to various techniques for achieving duplexing, i.e. reproducing the information contained in an original document on one or both sides of the copy sheet. However, none of the prior art appears to disclose any technique for producing copy sheets having areas color highlighted on one or both sides thereof. For example, no technique has been described wherein the first side may have a portion reproduced in black and a portion highlighted in red. Similarly, no approach has been developed for reproducing an original document such that both sides of the copy sheet have portion thereof in black and other portions thereof highlighted in red.
Accordingly, it is a primary object of the present invention to improve electrophotographic printing so as to create copies having color highlighted portions.